1. Field of the Invention
The invention relates to membrane exchangers such as those used for oxygenating blood, and heat exchangers such as those used to adjust the temperature of extracorporeally circulating blood during heart surgery.
2. Description of the Related Art
During open heart surgery, natural cardiovascular activity is suspended, which causes the lungs to collapse. It is, therefore, necessary to simulate the function of the lungs, which replace carbon dioxide in the blood with oxygen. Blood oxygenators serve this function. The typical blood oxygenator includes a membrane that acts as a boundary between extracorporeal blood flow and oxygen flow. As blood and oxygen pass over opposite sides of the membrane, oxygen passes through the membrane and into the blood and carbon dioxide passes, in the opposite direction, through the membrane and into the oxygen stream.
A heat exchanger may have a similar structure except that rather than employing a membrane boundary, the boundary layer is made of a non-permeable heat conducting layer. In use, blood is passed over one side of the boundary layer and a heat exchange fluid is passed over the other. In this manner a heat exchange occurs and the temperature of blood is thereby adjusted at a desired value.
Membrane exchangers and heat exchangers have various biomedical and non-medical uses in addition to blood oxygenating and cooling, and therefore the scope of this application is not limited to oxygenation and blood cooling.
A typical membrane exchanger includes an elongated sheet of membrane material folded into a plurality of pleats and sealed within a casing. Internal compartments are located on opposite sides of the casing for receiving the flap ends of the sheet material. A urethane sealant is introduced into the internal compartments to form a seal between the sheet material and the casing. Similarly, urethane resin seals the sheet material to other internal portions of the casing in order to direct fluid flow and prevent blood leakage across the boundary layer.
There are a number of drawbacks with related art devices. First, while urethane resins are generally recognized as being biocompatible, it is preferable to minimize the number of materials with which extracorporeally circulating blood comes into contact. In addition, when urethane resins are used as seals, the resin may become absorbed to varying degrees across individual folds in the sheet material. This can lead to very slight pockets of stagnated blood and can also cause differences in performance characteristics between similar exchangers.
Another problem with the use of urethane resin is that it requires a curing time of at least one and a half hours before the unit can be tested. Due to the critical nature of the fluid pathway integrity in a blood oxygenator, each unit is typically individually tested at the end of the manufacturing process. From urethane injection and the testing increases considerably the overall manufacturing process time.
Another drawback of related art devices is their size and weight. The use of urethane resin as a sealant adds considerable weight to the exchanger. In fact, the amount of urethane resin that is necessary to seal an exchanger weighs nearly as much as all of the other components of the exchanger combined. In addition, the internal compartments for receiving the ends of the sheet material and allowing a flow path for the urethane add size to the oxygenator. An oxygenator designed with internal compartments for sealing the ends of the sheet material also wastes material because, typically, 3/8 of an inch of material is sealed in resin on each side of the exchanger.